1. Field of the Invention
The present invention relates to a mode identifying circuit for use in a magnetic recording and/or reproducing apparatus capable of selective operation in a low-band mode and a high-band mode.
2. Description of the Prior Art
In a rotary VTR of the type in which a luminance signal is FM-modulated and a chroma signal is down-converted to a low frequency region for recording in slant tracks on a magnetic tape, horizontal resolution can be improved, and consequently high picture quality can be achieved, by increasing the carrier frequency that is frequency modulated by the luminance signal. Therefore, it has been recommended that a VTR be provided with a mode in which recording is carried out with a carrier frequency being used for frequency modulation with the luminance signal that is higher than the conventionally employed carrier frequency in such a VTR.
Thus, in the recording format of an 8-mm VTR, the carrier frequency for the FM modulation by the luminance signal is set conventionally to be 4.2 MHz at the sync tip level and 5.4 MHz at the white peak level, and it has been suggested that recording be performed with a high carrier frequency, that is, with a carrier frequency which is 5.7 MHz at the sync tip level and 7.7 MHz at the white peak level.
If such mode in which the carrier frequency for the frequency modulation by the luminance signal is increased, hereinafter called the "high-band mode", is to be selectively established in a VTR in addition to a conventionally employed mode which is hereinafter called the "low-band mode", in a reproducing operation of the VTR, it is necessary to change the characteristic of a luminance signal processing circuit in correspondence with the mode used when the tape was recorded. More specifically, it is necessary to change the sensitivity of an FM demodulator or a characteristic of a low-pass filter provided after the FM demodulator in accordance with whether the high-band mode or the low-band mode was used in recording the tape being played-back or reproduced. It is extremely troublesome to manually switch or change such characteristics of the VTR for the various modes, and, when changed manually, correct mode setting cannot be reliably achieved in the case where a mode transition occurs intermediate the ends of the tape.
For the above reasons, a VTR equipped with a mode identifying circuit has been proposed for automatically determining whether the video signal was recorded on the reproduced tape in the low-band mode or the high-band mode. Such a conventional mode identifying circuit detects a signal level of a predetermined frequency component which is concentrated in a recorded signal of one mode, and determines whether this detection level is equal to, or larger than a predetermined level as the basis for a mode identification.
An example of such conventional mode identification circuit is shown in FIG. 1 in which a reproduced frequency modulated (FM) luminance signal Y.sub.FM is supplied from an input terminal 100 to a band-pass filter 101 which passes a frequency component of a frequency f.sub.L which is included in the FM luminance signal Y.sub.FM and corresponds, for example, to the sync tip level of the video signal when recorded in the low-band mode. It will be appreciated that the frequency component which corresponds to the carrier frequency f.sub.L of the FM luminance signal for the sync tip level of the reproduced FM luminance signal Y.sub.FM can be picked up when reproducing a low-band recording. The output of the band-pass filter 101 is supplied to a detector 102 which detects level of the frequency component corresponding to the f.sub.L. The output of the detector 102 is applied to a negative input of a comparator 103 which receives a reference voltage at a positive input thereof. The comparator 103 detects whether the output level of the detector 102 is equal to or larger than the predetermined or reference voltage, and the resulting output of the comparator 103 is available at an output terminal 104 as a mode identifying output.
With the above-described example, if the reproduced FM luminance signal Y.sub.FM supplied to the input terminal 100 is a low-band signal, the output level of the detector 102 becomes equal to or greater than the predetermined or reference voltage. As a result, the output of the comparator 103 assumes a negative or low level. When the reproduced FM luminance signal supplied to the input terminal 100 is a high-band signal, it is intended that no signal equal to or larger than the reference voltage is detected at the detector 102, and that, as a result thereof, the output of the comparator 103 will assume a high level for identifying the high-band signal.
FIG. 2 shows another example of a conventional mode identifying circuit in which the reproduced FM luminance signal is applied from an input terminal 110 to a band-pass filter 111 which, for example, passes the frequency component f.sub.L corresponding to the sync tip level in the case of the low-band mode, and also to a band-pass filter 112 which, for example, passes a frequency component F.sub.h corresponding to the sync tip level in the case of the high-band mode. The output of the band-pass filter 111 is supplied to a detector 113, and the output of the detector 113 is applied to a negative input of a comparator 115. The output of the band-pass filter 112 is supplied to a detector 114, and the output of the detector 114 is applied to a positive input of the comparator 115. The resulting output of the comparator 115 is made available at an output terminal 116 as a mode identifying signal.
If the reproduced FM luminance signal supplied to the input terminal 110 is a low-band signal, usually the output level of the detector 113 is larger than the output level of the detector 114 so that the output of the comparator 115 is a negative or low level signal for identifying the low-band signal. When the reproduced FM luminance signal Y.sub.FM is a high-band signal, usually the output of the detector 114 is larger than the output of the detector 113 and, therefore, the output of the comparator 115 is at a high level to identify the high-band signal.
However, if the signal level of a predetermined frequency component concentrated in a reproduced FM luminance signal Y.sub.FM which had been recorded in a low-band mode, or the signal level of a predetermined frequency component concentrated in a reproduced FM luminance signal Y.sub.FM which had been recorded in a high-band mode is detected, as in FIGS. 1 and 2, and the reproducing mode is switched or changed-over in response to such detected level, there is a possibility that an erroneous mode identification will occur due to the influence of sideband components.
More specifically, in the conventional mode identifying circuit shown in FIG. 1, it is intended that an output equal to or greater than a predetermined value is obtained from the detector 102 only if the reproduced FM luminance signal Y.sub.FM was recorded in the low-band mode, whereas, if the reproduced FM luminance signal Y.sub.FM was recorded in the high-band mode, it is assumed that the output from the detector 102 will be clearly less than the predetermined value represented by the reference voltage supplied to the positive input of the comparator 103. However, in actual practice, even if the reproduced FM luminance signal Y.sub.FM was recorded in the high-band mode, its lower sideband may coincide with, or include the carrier frequency corresponding to the sync tip level of the low-band mode. In this case, a frequency component of the lower sideband is passed by the band-pass filter 101, and a signal equal to or greater than the predetermined level may be detected at the detector 102. Therefore, the output of the comparator 103 may assume a low level despite the fact that the reproduced FM luminance signal Y.sub.FM was recorded in the high-band mode, and it is erroneously indicated that the video signal being reproduced was recorded in the low-band mode.
Similarly, in the mode identifying circuit shown in FIG. 2, even when the reproduced FM luminance signal Y.sub.FM was recorded in the high-band mode, its lower sideband may be in the vicinity of the frequency f.sub.L characteristic of the carrier frequency for the sync tip level in the low-band mode and which is a pass band of the band-pass filter 111, and, if the level of the lower sideband at the frequency f.sub.L is larger than the level of the carrier at the frequency f.sub.H, the output level of the detector 113 is larger than that of the detector 114. As a result, the output of the comparator 115 assumes a low level so that there is an erroneous indication of the low band mode.
In order to prevent such erroneous mode indication, it has been proposed, for example, as shown in FIG. 3, to provide a gate circuit 106 between the input terminal 100 and the band pass filter 101 in FIG. 1 for transmitting the reproduced FM luminance signal only during each synchronization signal interval in response to a gating pulse GP suitably supplied to a terminal 107. Thus, the reproduced FM luminance signal Y.sub.FM is applied to the band pass filter 101 only during each synchronization signal interval through the gate circuit 106, and a predetermined frequency component of the reproduced FM luminance signal Y.sub.FM during each synchronization signal interval is detected for identifying the mode used for recording the reproduced video signal.
Since the level of the luminance signal is constant during each synchronization interval, the carrier of the FM luminance signal during each synchronization signal interval has a predetermined frequency and its sideband components are spread in a predetermined manner. As a result, by thus detecting the predetermined frequency component in the reproduced FM luminance signal Y.sub.FM only during each synchronization interval for identifying the recording mode, only a carrier component of the low-band mode can be detected, and an erroneous mode identification due to influence of the lower sideband component of a reproduced FM luminance signal recorded in the high-band mode can be prevented.
However, in order to detect the reproduced FM luminance signal Y.sub.FM only during the synchronization signal interval, it is necessary to develop the gate pulse GP corresponding to the synchronization period and supply the gate pulse GP to the terminal 107. The foregoing requires separation of a synchronization signal from a demodulated and reproduced luminance signal for the development of the gate pulse GP. However, an FM demodulator for demodulating the reproduced FM luminance signal Y.sub.FM cannot produce a correct demodulated output unless the demodulator is properly set in conformance with the mode used in recording. For this reason, at a time when the recording mode has not yet been identified, the synchronization signal cannot be separated from the reproduced FM luminance signal Y.sub.FM and the gate pulse GP cannot be readily formed.